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APPLIED SCIENCES

Breaks in Sitting Time

Effects on Continuously Monitored Glucose and Blood Pressure

BHAMMAR, DHARINI M.1,2; SAWYER, BRANDON J.1,3; TUCKER, WESLEY J.1,4; GAESSER, GLENN A.1

Author Information
Medicine & Science in Sports & Exercise: October 2017 - Volume 49 - Issue 10 - p 2119-2130
doi: 10.1249/MSS.0000000000001315

Abstract

Adults in the United States spend more than 50% of their waking hours sitting (39). Prolonged sitting is an independent risk factor for all-cause and cardiovascular mortality (6) and is associated with an adverse cardiometabolic risk profile (23), including higher blood pressure (18), impaired glucose tolerance, and increased risk for type 2 diabetes (49).

Interrupting sitting time with light- or moderate-intensity walking breaks lasting 1 min 40 s to 5 min has been shown to reduce postprandial glucose responses (2,13,24,46). Multiple short breaks of moderate-intensity (42,46) or moderate- to vigorous-intensity (26) activity may be more effective than a single continuous exercise session of equal total duration for reducing postprandial glucose concentrations. In fact, single sessions of 30 to 60 min of continuous exercise followed by prolonged sitting have been reported to have no effect on nonfasting blood glucose concentrations for up to 11 h after exercise (26,42,46,52). This is surprising because a single bout of exercise is known to increase rates of whole-body glucose disposal (20).

In contrast to published data on postprandial glucose, both multiple short bouts (3–10 min) and longer single sessions (30–40 min) of continuous moderate- to vigorous-intensity exercise reduce postexercise blood pressure compared with prolonged sitting, with multiple short bouts of exercise having an equal (40,41) or greater (1,4,45) effect. Light- and moderate-intensity walking breaks of just 2-min duration every 20 min over 5 h have been reported to reduce hourly blood pressure measurements during the 5 h of observation compared with sitting (33). Because that study did not compare the short bouts of physical activity to a longer continuous exercise session, it is unknown if these two methods of breaking up sitting time would elicit different blood pressure responses when compared with a single session of continuous exercise (33).

Another limitation of current published data on the impact of breaking up prolonged sitting is that measures of glucose and blood pressure were obtained primarily in laboratory settings during a relatively short period (i.e., 5–12 h), with glucose and blood pressure measured infrequently (e.g., hourly) (13,33). We have previously used ambulatory blood pressure (ABP) monitoring and continuous glucose monitoring (CGM), which allow for more frequent measurements under both laboratory and free-living trials, to show that breaking up sitting time with light- and moderate-intensity physical activity breaks of ≥10 min can reduce blood pressures and glucose over 24-h periods (4,10,53,54).

Therefore, the primary purpose of the present study was to examine the effects of multiple short walking breaks, each lasting 2 min, and one 30-min continuous walking session on interstitial glucose (measured by CGM) and ABP. We used two different multiple short-break protocols, one incorporating 2-min walks at a moderate intensity every 20 min (13,33) throughout a simulated workday in our laboratory, and one requiring participants to perform just one 2-min walk at a vigorous-intensity every hour. Energy expenditure was not matched across physical activity conditions because to do so would have required us to significantly reduce the duration and/or intensity of the 30-min continuous walking session and also the frequency and/or total number of 2-min moderate-intensity (2-min MOD) walks to match the low accumulated energy expenditure of the protocol requiring just one 2-min walk at a vigorous intensity every hour. This would have limited our ability to compare our results with previously published reports (1,4,13,32,46,52). Moreover, previous studies examining reductions in postprandial blood glucose after either low- or moderate-intensity walking breaks have shown no differences between the two trials despite differences in energy expenditure (13,33), suggesting that energy expenditure may not be the primary determinant of improvements in glucose control after physical activity breaks.

We hypothesized that, compared with uninterrupted sitting, interstitial glucose concentration and ABP would be lower during all physical activity trials, and that the two “multiple 2-min walk” protocols would produce lower glucose and ABP compared with the single 30-min continuous walking protocol.

METHODS

This randomized crossover full-factorial study was approved by the Arizona State University Bioscience Institutional Review Board. All participants were given a detailed description of the protocol and provided written informed consent.

Participant Screening

Participants were recruited through emails and flyers posted on the Arizona State University campuses. All participants were overweight or obese (BMI ≥ 25 kg·m−2), nonsmoking, physically inactive adults, between the ages of 18 and 45 yr for men and 18 and 55 yr for women. During a telephone interview, participants were asked “Do you exercise regularly (vigorous aerobic or resistance exercise) for more than 20 min·d−1 or are you currently training for an athletic competition?” and “Are you taking medications for high cholesterol, diabetes, blood pressure or heart disease.” Those who responded “yes” were excluded from the study. Additionally, participants who answered “yes” to any of the seven questions on the physical activity readiness questionnaire (7) or who had resting blood pressure greater than 140/90 mm Hg were excluded. All women were premenopausal and not pregnant. Women who did not have a history of regular menstrual cycles (i.e., variation of less than 8 d) were excluded.

Baseline Assessments

On the first laboratory visit, height was assessed using a stadiometer and weight was measured on a standard beam scale (Metric Weighing Scale; Detecto, Webb City, MO). Body fat was measured using the BodPod (LMI, Concord, CA). Standard prediction equations were used for estimating thoracic lung volume based on age, gender, height, weight, and ethnicity of the participant. Resting BP was measured once in each arm and then twice in the left arm with an automated BP monitor (Dinamap® PRO 100 Vital Signs Monitor; GE Healthcare, Little Chalfont, UK) on two separate days using standard procedures (8).

Maximal oxygen uptake (V˙O2max) was determined using a progressive continuous modified Balke protocol on a motorized treadmill. Participants began walking at 3.3 mph (88.5 m·min−1), 0% grade for the first minute. Grade was then increased to 2% for 1 min and by 1% increments every subsequent minute. After a 25% grade was reached, velocity was increased by 0.5 mph (13.4 m·min−1) every minute until the point of volitional exhaustion. Verbal encouragement was given to all participants during the test.

Pulmonary ventilation and gas exchange were monitored continuously for determination of V˙O2 (True One 2400 Metabolic Measurement System; Parvo Medics, Inc., East Sandy, UT). Standard three point calibration was performed before each test. HR was measured continuously using an HR monitor (Polar Electro Inc, Lake Success, NY). V˙O2max was defined as the highest 15-s average achieved during the test.

Trial Conditions

Participants returned to the laboratory to complete four trials, with each trial taking place over three consecutive days (Fig. 1). On the first day of each trial visit, participants were fitted with a continuous glucose monitor (described later). On the second day, participants arrived at the laboratory in the morning between 08:00 and 09:00 h, and stayed until 18:00 h to simulate a typical workday. This included a 1-h break for lunch, during which time participants remained seated. They were fitted with the ABP cuff and two readings of resting blood pressure were taken 5 min apart after a 15-min seated rest period. One of four trials, performed in random order and described in detail below, was conducted. For the three trials involving physical activity, HR was monitored continuously during each activity period. Participants were allowed to use the restroom whenever they needed to. Participants returned to the laboratory between 08:00 and 10:00 h the following morning for removal of the CGM and ABP monitor. The four trials were performed at least 72 h apart (average duration between trials was 7 d) to limit any potential carry-over effects. Female participants completed these visits during the follicular phase of the menstrual cycle to minimize variation in blood pressure that may be caused by the menstrual cycle (30).

F1-20
FIGURE 1:
Timeline for study participants.

Prolonged sitting

The control trial was a no-exercise, prolonged sitting (SIT) trial where participants were free to use the internet, work on their computer, or read during the 9 h spent in the laboratory, with breaks to use the restroom whenever needed as during a typical workday.

30 min of submaximal moderate-intensity walking

Participants performed 30-min of continuous exercise on a motor-driven treadmill, commencing at 12:00 h. The 12:00 h start was to maximize the chances of eliciting a postexercise hypotension (1,4). After a 3-min warm-up at 50% to 60% of HRmax, participants walked for 30 min at 3.3 mph (88.5 m·min−1) on a grade that elicited 65% to 75% of HRmax. This was followed by a 2-min cool-down at 2.5 mph (67.1 m·min−1) on a level grade. HR during the 30-min walk averaged 71% of HRmax, which corresponds to approximately 56% of V˙O2max (17). Estimated energy expenditure for the 30-min exercise session, including the warm-up and cool-down, was approximately 230 kcal.

2-min of moderate-intensity walking every 20 min

Participants performed 21 2-min bouts of moderate-intensity walking, once every 20 min, on a motorized treadmill at 3.0 mph (80.5 m·min−1) on a level grade. The first 2-min bout started at approximately 09:50 h and participants received a 1 h break for lunch. Based on data from Sawyer et al. (47), walking at 3 mph (80.5 m·min−1) for a total of 42 min would result in a cumulative energy expenditure of approximately 240 kcal.

2-min of vigorous-intensity walking every hour

Participants performed eight 2-min bouts of vigorous-intensity walking, once every hour over a period of 8 h. The 2-min bouts started with treadmill speed at 3.0 mph (80.5 m·min−1). Treadmill grade was then increased gradually over 40 s until the participant reached the maximal incline achieved during the V˙O2max test. Participants walked at this speed and incline for 60 s. Treadmill incline and speed were reduced over the next 15 s and the treadmill was stopped at 2 min. The first 2-min bout started at approximately 09:50 h. HR during these 2-min high-intensity walks averaged 79% ± 4% HRmax during the last 30 s, which corresponds to approximately 68% of V˙O2max (17). Cumulative energy expenditure for the eight 2-min high-intensity walks was estimated to be approximately 140 kcal.

Aside from the structured exercise during each of the three trials involving treadmill walking, participants spent the remainder of the day in the laboratory sitting as in the control day.

Continuous Glucose Monitoring

Interstitial glucose was measured using the iPro2 CGM system with a Sof-sensor glucose sensor (Medtronic, Northridge, CA). The device was calibrated using a handheld One Touch Ultra 2 measurement system (Lifescan Inc., Milpitas, CA) four times during the monitoring period. As per manufacturer’s instructions, this calibration was performed when blood sugar was expected to be stable (1 h after insertion, before lunch, before dinner and upon waking the next morning). The calibration values were used to construct glucose curves based on interstitial glucose recordings averaged every 5 min using the associated software (Solutions Software, Medtronic, Northridge, CA). Area under the curve (AUC), incremental AUC (iAUC), and net iAUC were calculated using the trapezoidal method, as we reported previously (10). For iAUC and net iAUC, preprandial baseline values were calculated as the average of two glucose values obtained just before each meal. Glucose values that were below the baseline were not included in the iAUC analysis (30% of all postprandial measurements). Net iAUC was obtained by first calculating iAUC using the trapezoidal method for both positive and negative glucose increments and then subtracting the area below fasting level from the area above (16). Glycemic variability was quantified using mean amplitude of glycemic excursions (MAGE), continuous overall net glycemic action (CONGA), and SD. These measures were calculated using an automated macro in Microsoft Excel (25).

Ambulatory Blood Pressure

The Oscar 2 ABP System (SunTech Medical, Morrisville, NC) was used for ABP monitoring. The ABP monitor was programmed and an appropriately sized cuff was used for each participant. The monitor was programmed to measure BP every 15 min during the daytime and every 45 min between estimated bedtime and wake time. Participants received a verbal explanation regarding the process of ABP monitoring and were also given written instructions regarding the frequency of inflations and deflations, how to deflate manually, what to do about failed measurements and to keep the monitor attached at night (44). Participants were also instructed to perform their habitual daily activities, not to engage in formal exercise, and to relax and straighten the arm during the recording interval for daytime ABP monitoring. Participants were provided with an activity diary and were asked to document their hours of sleep, time at work, time at leisure activities, and any symptoms experienced during the monitoring period (44).

Activity Monitor

During all the trials, participants were supervised for the 9 h that they were in the laboratory. They were fitted with the Actigraph GT3X+ monitors (ActiGraph, Pensacola, FL) just before commencing the 2-min walking breaks or before consuming breakfast, depending on the trial, to measure activity levels. After leaving the laboratory, participants were given instructions to not engage in any structured exercise for the rest of the day. Freedson 1998 (15) accelerometer cut-points (i.e., sedentary, 0–99 counts per minute; light-intensity activities, 100–1951 counts per minute; moderate-intensity activities, 1952–5724 counts per minute, vigorous activities: >5724 counts per minute) and cadence cut-points by Tudor-Locke and Rowe (51) were used to estimate sedentary time and activity levels.

Meals

Participants ate identical meals for all four trials. They were provided with a fixed breakfast (bagel with cream cheese, yogurt, and juice; 678 kcal, 130 g (77%) carbohydrate, 10 g (13%) fat, and 10 g (10%) protein), fixed lunch (frozen entree and chips; 518 kcal, 68 g (53%) carbohydrate, 18 g (31%) fat, and 21 g (16%) protein) and two fixed snacks (granola bars; 173 kcal, 19 g carbohydrate, 9 g fat, and 4 g protein) at the laboratory. For all trials, breakfast was provided at approximately 09:55 h and lunch was provided at approximately 13:50 h. Participants were also provided with Subway® gift cards for dinner on the night before each laboratory visit and for dinner after leaving the laboratory on the day of each experimental trial. Participants were given the option of ordering a 6-inch or 12-inch Subway® sandwich as long as they ordered the exact same sandwich every visit. The energy value of a foot-long Subway® sandwich with chips was estimated at 1014 kcal (148 g (58%) carbohydrate, 22 g (20%) fat, and 56 g (22%) protein). Participants were instructed to eat dinner at a similar time for all four trials, which they reported consuming on average between approximately 19:25 and 20:05 h. Twenty-four–hour energy intake was estimated at 2383 kcal, which included 365 g carbohydrate (61%), 59 g fat (22%), and 98 g protein (16%).

Power Calculation

An a priori power analysis was performed to determine the sample size necessary to detect significant changes in ABP and CGM. From previous ABP data collected at our laboratory (4), it was determined that seven participants would need to complete this within-subjects repeated measures study to detect differences in 24-h ABP between trials (α = 0.05; β > 0.80; variance explained by effect = 1.5; variance within group = 9.0; Cohen f = 0.4; F tests) (9,14). Based on data from Gillen et al. (19), it was determined that to detect differences in 24-h interstitial glucose (α = 0.05; β > 0.80; Cohen dz = 0.75; based on t test), 16 participants would need to complete the study. However, previous studies (10,37,38), including Gillen et al. (19), have achieved adequate power for detecting differences in interstitial glucose with 7 to 10 participants using a repeated measures design. Therefore, we determined that a sample size of 10 participants would allow us to achieve adequate power to detect differences in mean systolic BP and mean interstitial glucose.

Statistical Analysis

Data have been expressed as means ± SD unless otherwise specified. All P values were calculated assuming a two-sided alternate hypothesis; P < 0.05 was considered statistically significant. Q–Q plots as well as measures of skewness and kurtosis were used to test for normal distribution of the outcome variables. Baseline differences in mean glucose, systolic and diastolic BP, and mean arterial pressure (MAP) between trials were assessed with a repeated-measures ANOVA. The main outcomes of interest were mean glucose, glucose AUC, postprandial iAUC, net iAUC, BP, MAP, and physical activity levels. Linear mixed models were used to compare mean differences in outcomes between the four trials because it has been recommended as the appropriate statistical analysis tool for handling ecological momentary assessment data from CGM or ABP (28,48). A first-order autoregressive model was used to handle the range of time series patterns seen with CGM and ABP data (36). Age and sex were included as covariates in the model. If there was a main effect of trial, a Bonferroni correction was applied to post hoc analyses. Data were analyzed for four different periods, including the entire 18.7-h measurement period, during the laboratory phase (LAB; until 18:00 h), during the evening (EVE; 18:00–23:00 h) and during sleep (SLEEP; 23:00–07:00 h). For ABP data, SLEEP hours for each condition were determined based on self-report bedtime and waketime. For AUC calculations from CGM data, SLEEP hours were standardized across conditions for each participant by calculating the average bedtime and waketime and using this for each of the four conditions as done in a previous study (10). Because the first walking bouts started at different times (09:50 h for the multiple 2-min routines and 12:00 h for the 30-min continuous exercise session), data were analyzed separately starting from approximately 09:40 h (i.e., 10 min before the first 2-min exercise sessions) and from approximately 12:30 h (i.e., end of the 30-min exercise session). There were no differences in results for the two start points and results are presented for data collected after both 09:40 h and 12:30 h start times. SPSS software (SPSS 22; IBM Corporation, Armonk, New York, NY) was used for all statistical analyses.

RESULTS

Fifty-two volunteers responded to the recruitment materials and 36 completed the screening questionnaire. Only 12 volunteers met all inclusion criteria. Two participants dropped out due to time constraints. Therefore, 10 participants completed all four trials (Table 1).

T1-20
TABLE 1:
Participant characteristics (means ± SD).

Intensity During Exercise Trials

Relative exercise intensity (expressed as a percent of HRmax) was highest for the 2 min of vigorous-intensity walking every hour (2-min VIG) protocol (79% ± 4% HRmax during the last 30 s), followed by the 30 min of continuous moderate-intensity (30-min MOD) walking (71% ± 4% HRmax; P = 0.024 compared with 2-min VIG). The lowest exercise intensity was during the 2-min MOD protocol (53% ± 5% HRmax; P < 0.01 compared with the other two protocols). Although 53% of HRmax is considered very light (14), walking at 3.0 mph requires approximately 13 to 14 mL·kg−1·min−1 (47) and corresponds to approximately 3.7 to 4.0 METs, which is classified as moderate intensity.

Interstitial Glucose

There were more than 20% missing CGM values in six of the 40 trials that were conducted. The AUC values from these six trials were not included in the analyses for the overall as well as the postprandial periods. Two of these trials were from the same participant. Excluding this participant from the final analyses did not change the results (data not reported). However, the trials with missing data were included in the 18.7-h glucose analysis because mixed models have the ability to adjust for missing data (31). Moreover, excluding these trials did not change the final results (data not reported).

There were no significant differences in baseline fasting glucose values between the four trials (P = 0.253; Table 2). During the 18.7-h measurement period commencing with the completion of the 30-min MOD (≈12:30–07:20 h), glucose concentrations were significantly lower for all three exercise trials when compared with SIT (Table 2 and Fig. 2). Moreover, the 30-min MOD trial yielded lower mean 18.7-h glucose levels than both 2-min MOD and 2-min VIG protocols. Glucose was lower during 2-min MOD compared with 2-min VIG, regardless of whether the analysis started at 12:30 h or was expanded to include the morning hours (i.e., 09:50 h start time; Table 2). For the 30-min MOD, 2-min MOD, and 2-min VIG trials, lower glucose was observed during LAB, EVE, and SLEEP when compared with SIT, with the 30-min MOD trial achieving greater glucose reductions during EVE and SLEEP compared with 2-min MOD and 2-min VIG.

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TABLE 2:
Mean ± SD for interstitial glucose concentrations, AUC, iAUC, net iAUC, and measures of glycemic variability for 30-min MOD, 2-min MOD walking bouts every 20 min, 2-min VIG walking bouts every hour, and the SIT trial.
F2-20
FIGURE 2:
Mean interstitial glucose profiles for 30-min MOD, 2-min MOD every 20 min, 2-min VIG every hour and the SIT trial. See text and Table 2 for details and statistical comparisons.

There was a significant sex–trial interaction for differences in mean interstitial glucose. Data were analyzed separately for men (N = 5) and women (N = 4). One participant was excluded from these analyses because she had more than 20% missing CGM values for two trials. During the 18.7-h measurement period commencing with the completion of 30-min MOD, women achieved 6%, 13%, and 13% mean reductions in glucose for 30-min MOD, 2-min MOD and 2-min VIG, respectively, compared with SIT (5.4 ± 0.8 mmoL·L−1; P < 0.001 for all three comparisons). During the same period, men achieved a significant 3% mean reduction in glucose for 30-min MOD and 6% and 7% increases in mean glucose for 2-min MOD and 2-min VIG, respectively, compared with SIT (5.4 ± 0.8 mmoL·L−1; P < 0.001 for all three comparisons). With the exception of the 2-min MOD during LAB, where both women and men had 4% to 5% lower mean glucose compared to SIT, lower mean glucose for women and higher mean glucose for men were observed during EVE and SLEEP for 2-min MOD and during LAB, EVE and SLEEP for 2-min VIG.

After breakfast, mean 2-h postprandial glucose was significantly lower during the 2-min MOD and 2-min VIG trials compared with SIT (P < 0.001; Table 2 and Fig. 3A). After lunch and dinner, mean 2-h postprandial glucose was significantly lower during all three walking trials when compared with SIT (P < 0.001; Table 2 and Fig. 3B and 3C). In addition, mean 2-h postprandial glucose after the dinner meal was significantly lower during 30-min MOD and 2-min VIG trials compared with 2-min MOD (P < 0.001).

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FIGURE 3:
Postprandial mean interstitial glucose profiles for breakfast (A), lunch (B), and dinner (C) for 30-min MOD, 2-min MOD every 20 min, 2-min VIG every hour and the SIT trial. The 30-min MOD data were not included in Panel A because breakfast was provided before the 30-min exercise session. Mean interstitial glucose values are displayed every 5 min and error bars representing ± 1 S.D. are displayed every 20 min instead of every 5 min for clarity. *Significantly better than SIT (P < 0.05); ¶Significantly better than 2-min MOD.

There was a significant main effect of trial (i.e., 2-min MOD, 2-min VIG, and SIT) on 6-h postprandial glucose iAUC (P = 0.039), however post hoc tests did not reveal significant differences (i.e., P > 0.05) between trials (Table 2). There were no differences in glucose AUC overall, during LAB, EVE, SLEEP, or the iAUC or net iAUC during the remaining postprandial periods (Table 2). There were also no differences in measures of glycemic variability (MAGE, CONGA, SD) between the four trials (Table 2).

Ambulatory Blood Pressure

ABP data from three participants were excluded from the analysis as a result of >20% missing ABP data points overall or ≥5 h of missing data for two or more trials. ABP data from one additional participant was excluded because that participant consumed caffeinated drinks within 24 h of the start time of two trials. Data have been reported for the six remaining participants (four men and two women).

Systolic, diastolic and mean ABP are presented in Table 3. The SLEEP period represents self-reported sleep h, which were approximately between 23:00 and 07:00 h for most participants. There were no baseline differences in BP between the trials. Compared with SIT, only the 30-min MOD trial reduced 18.7-h systolic ABP and MAP (P < 0.05), which was primarily due to a significant reduction in systolic and mean ABP during EVE (but not during LAB or SLEEP). Compared to SIT, no walking trial produced a lower diastolic ABP during any period.

T3-20
TABLE 3:
Mean ± SD for systolic and diastolic BP, and MAP for 30-min MOD, 2-min MOD every 20 min, 2-min VIG every hour, and the SIT trial.

Activity Monitor

Three participants had missing data during LAB hours for one trial each because the Actigraph was not attached at the beginning of trial and these data were excluded from the analysis. LAB hour (464 ± 31 min) were conducted under supervision as stated above. During LAB hours of the SIT trial, participants spent 83% ± 8% (95% CI, 77%–89%) of their time engaged in sedentary behavior and 16% ± 8% (95% CI: 10%–22%) engaged in light-intensity activity (15). Using cadence data for LAB hours of the SIT trial, participants spent 95% ± 3% of the LAB hours with a cadence < 20 steps per minute, which included 0 steps per minute of nonmovement and 1 to 19 steps per minute of incidental activity (51). This was higher than 30-min MOD (87% ± 2%; P = 0.015) and 2-min MOD (84% ± 1%; P < 0.001) but not different from 2-min VIG (92% ± 3%; P = 0.305).

The Actigraph was able to detect moderate-intensity activity during the three walking conditions (i.e., 35 ± 5 min for 30-min MOD, 36 ± 6 min for 2-min MOD, and 14 ± 2 min for 2-min VIG) and significantly lower levels during SIT (4 ± 8 min; P < 0.001). Participants spent more time sedentary during SIT (395 ± 39 min) compared with 2-min MOD (343 ± 25 min; P = 0.010 main effect between trials; P = 0.050 post hoc test). There were no differences in sedentary time between SIT and 30-min MOD (334 ± 48 min; P = 0.288) or 2-min VIG (381 ± 24 min; P = 1.000). There were no differences in time spent in light- (79 ± 31 min; P = 0.772) or vigorous-intensity (1 ± 3 min; P = 0.103) activities between the four trials.

Three participants had no ActiGraph data during EVE hours for one or more trials and these data were excluded from the analysis. There were no significant differences in the number of minutes that participants spent in sedentary, light-intensity, or moderate-intensity activity outside the laboratory between the four trials (P = 0.491, 0.357, and 0.500, respectively). On average, participants spent 99.2% of their ambulatory hours outside the laboratory in sedentary (0–99 counts per minute) and light-intensity activities (100–1951 counts per minute), less than 1% of their time in moderate-intensity activities (1952–5724 counts per minute), and 0% of their time in vigorous activities (>5724 counts per minute).

DISCUSSION

Our results only partially supported our hypotheses regarding glucose and did not support our hypothesis regarding ABP. Although all three walking trials resulted in significantly lower mean interstitial glucose concentrations during the 18.7-h CGM measurement period, and during all 2-h postprandial periods, the single session of 30 min of continuous walking reduced mean 18.7-h glucose by more than either of the two multiple 2-min walking protocols. Contrary to our hypothesis for ABP, only the 30-min walking trial was effective for reducing systolic ABP.

Glucose

The beneficial effects on postprandial glucose of breaking up prolonged sitting with short bouts of light-to-moderate-intensity walking have been demonstrated and highlighted in a recent review (3). Many studies have examined blood glucose responses to breaks in sitting time over relatively short periods (i.e., 5–9 h) (2,13,24,26,42,46). Our CGM data extend these findings to show that short, 2-min bouts of walking performed at a moderate-intensity every 20 min or at a vigorous intensity every hour can lower interstitial glucose concentrations for more than 12 h after the last 2-min walk, as demonstrated by significantly lower glucose concentrations during EVE and SLEEP.

Unlike previous findings demonstrating that multiple short bouts of exercise lasting 1.5 to 5 min were more effective for lowering blood glucose over 8–12 h than an equivalent amount of continuous exercise (26,42,46), our data show that multiple 2-min bouts of walking were not more effective than a single 30-min walk for reducing mean 18.7-h interstitial glucose concentrations. In fact, our data indicate the opposite, as the 30-min MOD trial resulted in the lowest mean 18.7-h glucose concentrations. The reason for the discrepant findings is not readily apparent, although our results for 30-min MOD are in agreement with observations that a single exercise session increases rates of whole-body glucose disposal (20). The reduction in glucose during our 2-min MOD and 2-min VIG protocols may in part be attributed to alteration of gene expression of numerous skeletal muscle genes, including those associated with glucose metabolism, which has been demonstrated with a protocol involving 2-min walking breaks of either light- or moderate-intensity every 20 min over 5 h (34).

Although the 30-min walk was statistically superior to both multiple 2-min routines, the clinical significance of the 0.1 mmol·L−1 difference in 18.7-h mean glucose between the 30-min MOD and 2-min MOD protocols is questionable. However, compared with 2-min MOD, mean glucose for the 30-min MOD protocol was reduced by twice as much during the cumulative 4-h postprandial period (lunch and dinner), and by 2.5 times as much during the dinner alone when compared with SIT.

Using CGM, we recently reported that, compared to uninterrupted sitting, accumulating 150 min of light-intensity walking (1.0 mph; 2 METs) or stationary cycling (2 METs) during an 8-h workday reduced 24-h mean glucose by 5% to 11% and cumulative 6-h postprandial glucose iAUC by 24% to 44% (10). In the current study, 2-min moderate-intensity walking breaks every 20 min (42 min total over 9 h) reduced mean 18.7-h glucose by approximately 9%. Even less frequent breaks of 2-min of vigorous-intensity walking every hour (16 min total over 8 h) reduced mean 18.7-h glucose by approximately 5%. These reductions in mean glucose are comparable to those we observed in our previous study that required approximately 3.5 to 9 times as much accumulated physical activity (10). Although we did not show significant reductions in 6-h postprandial iAUC when comparing either 2-min MOD or 2-min VIG to SIT, there was a significant main effect of trial, and the magnitude of iAUC reductions (i.e., 32%–42%) after 2-min MOD and 2-min VIG were similar to the 6-h postprandial iAUC reductions after accumulated physical activity we reported previously (10). Our previous study used light-intensity walking or cycling to ensure that the 150 min of accumulated activity could be performed at walking/cycling workstations without interruption in office work (10). Walking at higher intensities (and treadmill grade), such as we used in the current study, is not conducive to performing office work simultaneously. However, 2-min walking breaks, either at a moderate-intensity every 20 min or at a more-vigorous intensity just once per hour, may be suitable for worksite environments by requiring minimal time away from an office desk.

Our 2-min VIG protocol reduced interstitial glucose concentrations for nearly 14 h, including 5 h after the LAB phase (i.e., 18:00–23:00 h). The lack of an effect during sleep may be due to low amount of accumulated walking (16 min) throughout the LAB phase. Nevertheless, our finding that just one vigorous-intensity 2-min walk every hour for 8 h during a simulated workday was sufficient to reduce mean glucose during waking hours by approximately 7% may have clinical significance. Elevated nonfasting blood glucose is a risk factor for cardiovascular disease even in apparently healthy adults without diabetes (35), and postprandial hyperglycemia has been reported to better predict cardiovascular disease than fasting blood glucose in normoglycemic adults (11).

Separate analyses for men and women revealed that responses in women accounted for all of the improvement in 18.7-h mean glucose reductions during 2-min MOD and 2-min VIG, while both men and women responded with favorable reductions in glucose after the 30-min MOD. Exercise intensity is not a likely explanation since the relative intensity (% HRmax) during physical activity conditions was not different for men and women. This sex difference was evident even with a small sample size and is consistent with the findings of Dempsey et al. (12), who reported a significant sex–condition interaction in glucose responses during a trial consisting of intermittent walking breaks (3-min bouts of walking every 30 min) in inactive overweight or obese participants with type 2 diabetes. Compared with uninterrupted sitting, during the trial with intermittent walking breaks women experienced a 58% lower postprandial glucose net iAUC compared to a 26% lower glucose net iAUC for men (12). Possible mechanisms for sex differences in glucose responses after intermittent walking breaks need to be explored in future studies.

There were no differences in measures of glycemic variability such as MAGE, CONGA and SD between the four trials. All participants were nondiabetic and SD for interstitial glucose across the day for participants was approximately 0.6 mmol·L−1, suggesting low magnitude glucose excursions during the day. This may have limited the ability to detect differences in glycemic variability between trials. Future studies in diabetic participants may be able to address whether interrupting prolonged sitting can improve glycemic variability.

Ambulatory Blood Pressure

The lack of ABP reduction during each of the multiple 2-min walking protocols was unexpected. It has been reported that, compared to continuous sitting, light- and moderate-intensity walking breaks of 2-min duration every 20 min over 5 h reduced systolic and diastolic BP by 2 to 3 mm Hg during the 5 h of observation (33). Our virtually identical protocol extended over 8 h had no effect on ABP during any period over the 18.7 h of ABP monitoring. This was true even when we expanded our analysis to include the morning hours (i.e., before the 30-min exercise session was performed; data not reported). Since participants in Larsen et al. (33) had slightly higher blood pressures than our participants, including 5 of their 19 participants being hypertensive, it may have been easier to elicit BP reductions in that study. Furthermore, Larsen et al. (33) measured seated blood pressure once per h, always 5 min preceding each 2-min walking bout. Our ABP protocol included measurements every 15-min during waking hours, which may have been negatively influenced by the proximity of readings to the 2-min walks. However, this would not explain the absence of a reduction in ABP during EVE and SLEEP.

The most important difference between our study and that of Larsen et al. (33) is that we included a 30-min continuous walking session for comparison. The reduction in systolic ABP of 3 mmHg during the 18.7-h observation period after the 30-min MOD protocol is the same as we have reported previously (4). We elected to start the 30-min walking session midday to increase the chances of inducing a postexercise hypotensive response (1,4,29). The ABP-lowering effect of the 30-min MOD protocol was observed only during waking hours, and not during sleep, which is consistent with our previous findings (4).

The minimum duration for walking breaks to reduce ABP requires additional research. Multiple 10-min bouts at 1- to 4-h intervals during the day have been shown to reduce BP by more than a single exercise session lasting 30 to 40 min (1,4). Ten 3-min exercise sessions of moderate- or vigorous-intensity performed every 30 min have been shown to lower resting BP the following day by the same amount as a single 30-min exercise session (40,41). Notwithstanding the data of Larsen et al. (33), our results suggest that 2-min walking bouts are not sufficient stimulus to reduce ABP in healthy adults with relatively normal BP.

Strengths and Limitations

The use of CGM and ABP allowed us to examine glucose and blood pressure more frequently and over a longer period of time than has been studied previously (2,13,24,26,33,46), including not only the 9 h spent in the laboratory, but during evening hours after work and during sleep. The dietary intake of participants was tightly controlled during the study starting from dinner on the night before their simulated workday in the laboratory and lasting through dinner on the night of the laboratory visit. Since participants consumed the same meals for each trial, within-subject statistical comparisons were not possible for diet data. Meal size was not determined based on each individual participant’s energy needs. Participants were allowed to choose whether they wanted to eat a 6-inch or 12-inch Subway® sandwich at dinner and they were allowed to choose when to eat their granola snack bars as long as they were eaten at the same time of day for each trial. Nevertheless, it is possible that participants may not have been in energy balance since energy intake was not customized to achieve energy balance. The repeated measures design where each participant served as his or her own control reduces within-subject variation due to an energy surplus or deficit in the metabolic response to meals and increases confidence that the differences between trials were attributable to the exercise or activity breaks instead of diet.

Actigraph data confirmed that physical activity outside the LAB phase was not different between trials, and consisted of >99% sedentary and/or light-intensity activity. Consequently, it is unlikely that differences in glucose and ABP outside the laboratory were due to compensatory behavior during the evening hours. Actigraph data confirmed that participants were sedentary during LAB phase for all trials except while they were engaging the planned activity bouts. During the SIT trial, participants were sedentary for 83% of the LAB h, which is consistent with estimates of sedentary time (i.e., 0–99 counts per minute) in office workers (50). However, office workers typically spend 80% of time sitting (21) and in the current study, over 95% of LAB hours during SIT were spent in nonmovement or incidental movement suggesting that the SIT trial may not accurately represent behavior of typical office workers. However, the conditions of SIT in the current study were similar to other studies that have examined the effects of walking breaks on glucose control and blood pressure (2,10,12,13). Also, we were not able to estimate posture allocation from Actigraph data and thus studying the effects of posture-allocation on glucose control and ABP were not possible.

Since we did not objectively monitor activity levels for the 48 h before the start of each trial, we cannot be completely certain that participants had similar activity patterns on the days before each trial. However, we did instruct participants to avoid any vigorous exercise for at least 48 h before their visit. The fact that our participants were habitually sedentary reduces the likelihood that exercise before each visit influenced the results. Moreover, baseline values for glucose and BP were not different between trials.

Due to missing data from three participants, and noncompliance to dietary restrictions for one participant, our ABP analyses were reduced to only six participants. Despite this, data for the 30-min MOD protocol were similar to our previously published data using the same protocol, suggesting that a sample size of 6 was adequate to detect differences between 30-min MOD and SIT trials, which was consistent with our sample size calculation. Additionally, inspection of ABP data for both 2-min MOD and 2-min VIG protocols suggests that these protocols had no effect on any ABP measures (~0–1 mm Hg differences in almost all comparisons; see Table 3; and see Figure, Supplementary Digital Content 1, mean systolic, diastolic and mean arterial pressure responses for all trials, https://links.lww.com/MSS/A950) and that a larger sample size may not have substantially affected these results. Nevertheless, the small sample size in the current study affects the generalizability of the results and future studies should examine the effects of interrupting sitting on ABP using a larger sample size.

Energy expenditure across the three walking trials was not matched. This would have been nearly impossible to achieve, due to our inclusion of the 2-min VIG protocol requiring only one 2-min walk every hour (~140 kcal). We felt that this protocol was the most novel aspect of our study. To match energy expenditure it would have required us to reduce our 30-min walk (~230 kcal) to approximately 18 min, which would have prevented us from comparing our results with other studies that used 30-min exercise sessions (42,46,52). Moreover, the finding that the single 30-min walk was more effective for reducing both glucose and ABP, despite lower total energy expenditure than 2-min MOD, suggests that energy expenditure alone (within the range studied) is not a critical factor. These findings are consistent with reports that showed similar reductions in postprandial blood glucose (13) and seated BP (33) after breaks in sitting time that were of either low- or moderate-intensity walking, with no differences between the two trials even though they differed in total energy expenditure.

Postmeal exercise is recommended over pre-meal exercise for improving glucose control based on limited data in different populations (22). Our 30-min MOD walking session was conducted 150 min after breakfast and 30 min before lunch. Previous studies that have had women perform light-intensity exercise immediately after breakfast showed attenuation of postlunch rise in glucose (27,43). Other studies have conducted exercise sessions 120 min after breakfast with mixed results on glucose control (5,38). Charlot et al. (5) showed lower levels of glucose during the 75 min of vigorous-intensity exercise but not postlunch in young active men and Little et al. (38) showed no reduction in postlunch glucose AUC but significant reduction in postdinner glucose AUC after 30 min moderate-intensity exercise in obese adults. It is possible that the findings of the current study could have been different if the 30-min MOD had been conducted immediately after breakfast. In addition, by conducting the 30-min MOD in the afternoon, we were not able to conduct head-to-head comparisons of the walking breaks versus the 30-min MOD for the time between breakfast and lunch. However, the timing of the 30-min MOD was chosen to maximize the postexercise hypotensive response and future studies may consider conducting 30 min of moderate-intensity exercise closer to breakfast to examine the effect of timing of exercise on glucose control. Finally, this study had a small sample size and the results of this study can only be generalized to healthy, overweight and obese adults and may not be applicable to older individuals, children or pregnant women. Future studies with larger sample sizes should be conducted to assess the effects of interrupting prolonged sitting on glucose control and blood pressure.

CONCLUSIONS

Our results demonstrate that a single 30-min walk is more effective than breaking up prolonged sitting with 2-min walks at a moderate intensity every 2 min or 2-min walks at a vigorous intensity every hour for reducing glucose and ABP over an 18.7-h measurement period that included workday hours (LAB phase), EVE and SLEEP. Nevertheless, our data demonstrate that breaking up prolonged sitting with 2-min walking breaks, performed at a moderate-intensity every 20 min or at moderate-to-vigorous intensity every hour, significantly reduced mean 18.7 h and postprandial interstitial glucose concentrations. Although reductions in mean overall and postprandial glucose were not as great as those observed during a day that included just one 30-min walk, 2-min walking breaks may be more easily incorporated into a daily routine at office worksites. Our finding that just one 2-min walk every hour was sufficient to significantly reduce mean overall and postprandial glucose over an approximately 13-h monitoring period that included both the simulated workday phase and evening hours before sleep, highlights the potential benefit of very low doses of physical activity for improving glycemic control. Unlike the results for glucose, our data indicate that 2-min walking breaks are not effective for reducing ABP in adults with relatively normal BP.

This research was supported by a Graduate Research Support Program grant from the Arizona State University Graduate and Professional Student Association.

All authors declare that they have no competing financial interests related to this publication. All authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by ACSM.

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Keywords:

AMBULATORY; OVERWEIGHT; OBESITY; INTERMITTENT EXERCISE; PROLONGED SITTING; WALKING BREAKS

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© 2017 American College of Sports Medicine